feat(runtime): per-component Builder traits and BuilderContext

[mfw78]

Summary

Closes #90.

Introduces a per-component ComponentBuilder trait and a BuilderContext, plus concrete builders for the provider pool, local store, and (in the cow extension crate) the reference order book, assembled by a ComponentsBuilder that composition roots call instead of the previous four scattered open calls.

This is part of the epic #79 train and merges in stack order, on top of the injectable TaskExecutor from #89.

Changes

• Added BuilderContext<'a> { config: &EngineConfig, data_dir: &Path } and trait ComponentBuilder { type Output; fn build(self, ctx) -> impl Future<Output = anyhow::Result<Output>> + Send } in crates/nexum-runtime/src/host/component/builder.rs.
• Added core builders: ProviderPoolBuilder (wraps ProviderPool::from_config) and LocalStoreBuilder (creates data_dir and opens data_dir/local-store.redb), plus a no-op impl ComponentBuilder for () for the empty-extension lattice.
• Added ComponentsBuilder<C, S, E> whose build::<T: RuntimeTypes> drives the three builders against the Chain/Store/Ext lattice and sizes a fresh LogPipeline from limits.logs().
• Added ReferenceExtBuilder in crates/shepherd-cow-host/src/ext_cow.rs, wrapping OrderBookPool::from_config. This stays in the cow extension crate, not core, matching the existing crate boundary (OrderBookPool is owned by shepherd-cow-host; the core runtime carries no cow dependency).
• Updated crates/nexum-cli/src/launch.rs to build components via ComponentsBuilder::new(ProviderPoolBuilder, LocalStoreBuilder, ReferenceExtBuilder).build::<ReferenceTypes>(&ctx).
• Updated crates/nexum-runtime/examples/embed.rs to use the core-only lattice, ComponentsBuilder::new(ProviderPoolBuilder, LocalStoreBuilder, ()), retaining the embedder's log read handle via components.logs.clone().

Test plan

• cargo fmt --all --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --all-features --no-fail-fast (all green, zero failed, zero skipped)
• Boot and dispatch end-to-end tests confirmed running (not silently skipped), including e2e_supervisor_boots_example_module, e2e_balance_tracker_block_dispatch, e2e_ethflow_watcher_log_dispatch, e2e_stop_loss_block_dispatch, e2e_twap_monitor_block_dispatch, twap_monitor_without_cow_extension_fails_to_boot, and the cow_boot suite
• embed.rs example builds against the core-only (cow-free) lattice

Notes for reviewer

build is impl Future<Output = ...> + Send rather than a bare synchronous function, because ProviderPool::from_config genuinely dials WS transports and needs to be async. This follows the existing ChainProvider idiom elsewhere in the codebase and keeps builders usable from a spawned task; the synchronous builders (LocalStore, cow) simply have await-free bodies.

One ordering change worth flagging explicitly: previously create_dir_all(data_dir) ran before the provider pool was opened; now the provider pool builder runs first and LocalStoreBuilder creates the data directory as part of opening the store. Neither the provider pool nor the cow extension touches data_dir, and the directory is still created before the cow extension opens its own state, so this reordering is benign, but it is a real behavioural change from a pure move and is called out here rather than left implicit.

Follow-ups intentionally left out of scope for this PR:

• Threading ComponentsBuilder into the epic's declarative type-state RuntimeBuilder, so the builder assembly and the executor choice from supervisor: introduce TaskExecutor, thread it through stream openers #89 meet in one place rather than at the imperative composition roots.
• Threading a chosen TaskExecutor from the launcher rather than having bootstrap::run hard-pick TokioExecutor.
• A typed BuildError enum (thiserror) at the ComponentBuilder seam instead of anyhow, for structured boot-failure classification.
• Letting ComponentsBuilder optionally accept a pre-built LogPipeline so an embedder can inject a custom sink instead of always getting the in-memory default.

AI Assistance: Claude Code (Opus 4.8 implementation and adversarial review, Sonnet 5 PR authoring) used for the full change.